Page 109 - Accounting Information Systems

Appendix

Section A: Secondary Storage

computer’s secondary storage includes devices used to store and retrieve system software, appli-
cation software, and data on magnetic or optical media, such as magnetic tape, hard or floppy
A disks, and CD-ROMs.

Magnetic Tape

Most modern magnetic tape systems use reels that are similar to a VCR tape. A tape drive is used to re-
cord bits of data onto magnetic tape by winding the tape from one reel to the other and passing it across a
read/write head. The tape drive reads and writes blocks of data at a time. Each block is separated by an
interblock gap, which instructs the tape drive to stop reading or writing the data until another block is
requested. Figure 2-34 shows records blocked together on a magnetic tape.
A byte of data (representing a character, digit, or special symbol) is recorded on the tape across its
width. (One byte equals eight binary digits, or bits. A bit is the smallest possible unit of electronic information,
with a value of either 0 or 1.) A logical sequence of characters makes a field, and several fields make a record.
Although seldom used for data processing these days, magnetic tape still offers some important
advantages as a secondary storage medium. For example, large amounts of data can be stored on mag-
netic tape at a relatively low cost, and magnetic tape is reusable. The primary disadvantage is that tapes
record data sequentially, making data retrieval slower than direct access storage media. Modern tape sys-
tems alleviate this problem by using a form of indexing, in which a separate lookup table provides the
physical tape location for a given data block or by marking blocks with a tape mark that can be detected
while winding the tape at high speed.
Historically, tape has offered cost advantages over disk storage to make it a viable solution for data
backup. Rapid improvement in disk storage density, however, combined with sluggish innovation in tape
storage technologies, is eroding the market share of tape storage devices.

Magnetic Disks

The data stored on magnetic disks (hard disks or floppy disks) are considered nonvolatile. The data will
reside in a certain location on the magnetic surface until they are replaced with different data or erased.
Data can be recorded to magnetic disks using either of the access methods described earlier.
To get the disk ready to receive data, its surface must be formatted. An operating system utility pro-
gram formats the disk by dividing it into circular tracks and wedge-shaped sectors, which cut across the
tracks. The number of bytes that can be stored at a particular track and sector determines the disk’s density.
Disks are known as direct access storage devices because a piece of data can be accessed directly
on the disk. Database management systems and application software work with the operating system to
determine the location of the required data.
A disk has a rotating magnetic surface and a read/write head. The read/write head is on an access
arm that moves back and forth over the magnetic surface. The time that elapses from the request made of